For ignition timing control of the internal combustion engine, a technique has been proposed in which ignition timing is determined based on an operating state of the engine and the change in the rotation of the engine is detected as a change in angular signals until immediately before the actual ignition to set suitable ignition timing, as described in Japanese Examined Patent Publication 7-18395.
To further suppress the change in number of rotations or rotational speed of the engine in such ignition timing control, feedback control of the ignition timing may be performed with respect to the rotational speed of the engine. In this feedback control, base ignition timing, as calculated based on the operating state of the engine as described in Japanese Examined Patent Publication 7-18395, is corrected with a correction amount determined in the feedback control of the ignition timing.
However, such calculation for the feedback control takes a longer time compared to the control of Japanese Examined Patent Publication 7-18395 performed by correcting the ignition timing based on the detection of the change in angular signals. For this reason, the base ignition timing must be calculated at timing sufficiently before the piston comes to an ignition top dead center. Then, the calculation for the feedback control is performed based on the rotational speed of the engine to calculate the correction amount and the base ignition timing is corrected with the correction amount to obtain final ignition timing. Actual ignition is carried out after the final ignition timing elapses.
As described above, the final ignition timing is calculated at the timing sufficiently before the piston comes to the ignition top dead center. Unfortunately, this increases the potential for overshoot when the feedback gain is increased to achieve highly-responsive feedback control. This is because, when the period between the time when the final ignition timing was calculated and the actual final ignition timing becomes longer, the change in rotational speed of the engine during the period becomes considerable. The long waste time in the feedback control prevents effective suppression of the change in rotational speed of the engine.